BMPs as mediators of roof plate repulsion of commissural neurons

During spinal cord development, commissural (C) neurons, located near the d orsal midline, send axons ventrally and across the floor plate (FP). The tr ajectory of these axons toward the FP is guided in part by netrins. The mec hanisms that guide the early phase of C axon extension, however, have not b een resolved. We show that the roof plate (RP) expresses a diffusible activ ity that repels C axons and orients their growth within the dorsal spinal c ord. Bone morphogenetic proteins (BMPs) appear to act as RP-derived chemore pellents that guide the early trajectory of the axons of C neurons in the d eveloping spinal cord: BMP7 mimics the RP repellent activity for C axons in vitro, can act directly to collapse C growth cones, and appears to serve a n essential function in RP repulsion of C axons.